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Unified Diagnostic Services

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Unified Diagnostic Services (UDS) is a diagnostic communication protocol used by electronic control units (ECUs) in automotive electronics.[1] UDS is defined by ISO 14229 and evolved from ISO 14230 (KWP2000), which is now largely obsolete. UDS specifies functionality at the session, presentation, and application layers (layers 5–7) of the OSI model. Because of this, it can operate on different physical and data link layers such as CAN (ISO 11898), LIN (ISO 17987), Ethernet (ISO 13400), FlexRay (ISO 17458), and K-Line (ISO 14230). In practice, UDS is most commonly used over CAN via Diagnostic over CAN (DoCAN), defined in ISO 15765.

The term "unified" refers to the fact that UDS is an international standard rather than a manufacturer-specific protocol. Today, nearly all Tier 1 suppliers implement UDS in ECUs developed for automotive original equipment manufacturers (OEMs). UDS is also integrated into larger software architectures, including AUTOSAR.

Modern vehicles have a diagnostic interface for on-board diagnostics, which makes it possible to connect a computer (client) or diagnostics tool, which is referred to as tester, to the communication system of the vehicle. Thus, UDS requests can be sent to the controllers which provide responses (this may be positive or negative). This makes it possible to interrogate the fault memory of the individual control units, to update them with new firmware, have low-level interaction with their hardware (e.g. to turn a specific output on or off), or to make use of special functions (referred to as routines) to attempt to understand the environment and operating conditions of an ECU to be able to diagnose faulty or otherwise undesirable behavior.

Services

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SID (Service Identifier)[2]

Request Service Identifier (SID) Response Service Identifier (RSID) Service name Description
0x10 0x50 DiagnosticSessionControl[3] DiagnosticSessionControl service is used to change diagnostic sessions in the server(s). In each diagnostic session a different set of diagnostic services (and/or functionalities) is enabled in the server.

Server shall always be in exactly one diagnostic session.

0x11 0x51 ECUReset[4] ECUReset service is used by the client to request that the server perform a reset. The server, after receiving this request, performs the specified type of reset (either before or after transmitting the positive response).
0x14 0x54 ClearDiagnosticInformation[5] ClearDiagnosticInformation service is used by the client to clear Diagnostic Trouble Codes (DTCs) and related data stored in one or more server memories.
0x19 0x59 ReadDTCInformation[6] ReadDTCInformation service allows the client to request current Diagnostic Trouble Code (DTC) information from one or more servers within the vehicle.
0x22 0x62 ReadDataByIdentifier[7] ReadDataByIdentifier service allows the client to request data record values from the server identifier by one or more DataIdentifiers (DIDs).
0x23 0x63 ReadMemoryByAddress[8] ReadMemoryByAddress service allows the client to request server's memory data stored under provided memory address.
0x24 0x64 ReadScalingDataByIdentifier[9] ReadScalingDataByIdentifier service allows the client to request the scaling information associated with a Data Identifier (DID). Scaling data provides information required to correctly interpret the actual data value, such as:
  • data encoding type (e.g. integer, floating-point, ASCII)
  • units and formats
  • conversion formulas and coefficients
  • bit mappings, and other interpretation details
0x27 0x67 SecurityAccess[10] SecurityAccess service allows the client to unlock functions/services with restricted access.

Unlocking sequence:

  1. The client requests a seed from the server.
  2. The server responds with a positive response that includes a randomly generated seed value.
  3. Both the client and server compute a key value based on the seed (using a secret algorithm).
  4. The client sends the computed key to the server.
  5. The server validates the client by comparing the received key with its own calculated key. If they match, the client is granted access to the protected functionality for the corresponding security level.
0x28 0x68 CommunicationControl[11] CommunicationControl service allows the client to switch on/off the transmission and/or the reception of certain messages on the server(s).
0x29 0x69 Authentication[12] Authentication service provides a mechanism for the client to prove its identity, allowing access to data and/or diagnostic services that have restricted access due to security, emissions, or safety requirements.

It is an alternative for SecurityAccess service.

0x2A 0x6A ReadDataByPeriodicIdentifier[13] ReadDataByPeriodicIdentifier service allows the client to request periodic transmission of data record values from the server. Each periodic data record is identified by Periodic DID (the second byte of a DID with a fixed first byte 0xF2).
0x2C 0x6C DynamicallyDefineDataIdentifier[14] DynamicallyDefineDataIdentifier service allows the client to define Data Identifiers.
0x2E 0x6E WriteDataByIdentifier[15] WriteDataByIdentifier service allows the client to set data values stored under given Data Identifier.
0x2F 0x6F InputOutputControlByIdentifier[16] InputOutputControlByIdentifier service allows the client to take temporary control over Data Identifier values.
0x31 0x71 RoutineControl[17] RoutineControl service allows the client to execute any sequence of actions that are defined as routines. Each routine has Routine Identifier assigned.
0x34 0x74 RequestDownload[18] RequestDownload service allows the client to initiate data transfer from the client to the server.
0x35 0x75 RequestUpload[19] RequestUpload service allows the client to initiate data transfer from the server to the client.
0x36 0x76 TransferData[20] TransferData service transfers the actual data that were earlier requested by either RequestDownload, RequestUpload or RequestFileTransfer service.
0x37 0x77 RequestTransferExit[21] RequestTransferExit services ends the data transmission using TransferData service.
0x38 0x78 RequestFileTransfer[22] RequestFileTransfer service allows the client to initiate a file transfer (either download or upload).

It is an alternative for RequestDownload and RequestUpload services.

0x3D 0x7D WriteMemoryByAddress[23] WriteMemoryByAddress service allows the client to set values directly in the server's memory.
0x3E 0x7E TesterPresent[24] TesterPresent service informs server(s) that the diagnostic tester device (client) is connected and server(s) shall not exit their current diagnostic session.
0x83 0xC3 AccessTimingParameter[25] AccessTimingParameter was withdrawn in ISO 14229-1:2020.
0x84 0xC4 SecuredDataTransmission[26] SecuredDataTransmission service allows the client to secure UDS communication using either encryption or signature.
0x85 0xC5 ControlDTCSettings[27] ControlDTCSettings service allows the client to manage DTCs' statuses updating.
0x86 0xC6 ResponseOnEvent[28] ResponseOnEvent service allows the client to define events and request messages that the server is supposed to respond to when the event occurs.

This can be used to diagnose complicated issues. For example the client can request from the server to respond to ReadMemoryByAddress request (to provide detailed information about the reason for DTC reporting) when testFailed bit of a certain DTC is set.

0x87 0xC7 LinkControl[29] LinkControl service allows the client to change physical connection parameters.

As an example, for CAN bus, the client could change baudrate and version of CAN protocol (e.g. from CAN-FD to Classical CAN).

Diagnostic Message

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There are two types of diagnostic messages transmitted using UDS protocol:

  • Request
  • Response

Request Message

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Request messages are transmitted by a Client towards one or more Servers.[30]

Service Identifier (SID) is the first byte in each request message.[31]

Response Message

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Response messages are transmitted by Servers to a Client.[32]

The first byte in the response message is usually Response Service Identifier (RSID) value (with the exception of following responses to ReadDataByPeriodicIdentifier service).[33][13]

Positive Response Message

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Format of each positive response message is specific for the diagnostic service it relates to.

Negative Response Message

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Negative Response Message format
Byte Value Description
#1 0x7F Negative Response SID
#2 SID SID value from request message
#3 NRC reason for the rejection
Negative Response Codes
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Negative Response Codes (NRC in short) carry the information for request message rejection.[34]

Negative Response Codes[34]
Value Name
0x10 generalReject
0x11 serviceNotSupported
0x12 SubFunctionNotSupported
0x13 incorrectMessageLengthOrInvalidFormat
0x14 responseTooLong
0x21 busyRepeatRequest
0x22 conditionsNotCorrect
0x24 requestSequenceError
0x25 noResponseFromSubnetComponent
0x26 FailurePreventsExecutionOfRequestedAction
0x31 requestOutOfRange
0x33 securityAccessDenied
0x34 authenticationRequired
0x35 invalidKey
0x36 exceedNumberOfAttempts
0x37 requiredTimeDelayNotExpired
0x38 secureDataTransmissionRequired
0x39 secureDataTransmissionNotAllowed
0x3A secureDataVerificationFailed
0x50 Certificate verification failed, Invalid Time Period
0x51 Certificate verification failed, Invalid Signature
0x52 Certificate verification failed, Invalid Chain of Trust
0x53 Certificate verification failed, Invalid Type
0x54 Certificate verification failed, Invalid Format
0x55 Certificate verification failed, Invalid Content
0x56 Certificate verification failed, Invalid Scope
0x57 Certificate verification failed, Invalid Certificate (revoked)
0x58 Ownership verification failed
0x59 Challenge calculation failed
0x5A Setting Access Rights failed
0x5B Session key creation/derivation failed
0x5C Configuration data usage failed
0x5D DeAuthentication failed
0x70 uploadDownloadNotAccepted
0x71 transferDataSuspended
0x72 generalProgrammingFailure
0x73 wrongBlockSequenceCounter
0x78 requestCorrectlyReceived-ResponsePending
0x7E SubFunctionNotSupportedInActiveSession
0x7F serviceNotSupportedInActiveSession
0x81 rpmTooHigh
0x82 rpmTooLow
0x83 engineIsRunning
0x84 engineIsNotRunning
0x85 engineRunTimeTooLow
0x86 temperatureTooHigh
0x87 temperatureTooLow
0x88 vehicleSpeedTooHigh
0x89 vehicleSpeedTooLow
0x8A throttle/PedalTooHigh
0x8B throttle/PedalTooLow
0x8C transmissionRangeNotInNeutral
0x8D transmissionRangeNotInGear
0x8F brakeSwitch(es)NotClosed (Brake Pedal not pressed or not applied)
0x90 shifterLeverNotInPark
0x91 torqueConverterClutchLocked
0x92 voltageTooHigh
0x93 voltageTooLow
0x94 ResourceTemporarilyNotAvailable

See also

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References

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  1. ^ "Iso 14229-1:2020 Unified diagnostic services (UDS) Part 1: Application layer".
  2. ^ Dąbrowski, Maciej. "Diagnostic Service — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-09.
  3. ^ Dąbrowski, Maciej. "Diagnostic Service (DiagnosticSessionControl) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-09.
  4. ^ Dąbrowski, Maciej. "Diagnostic Service (ECUReset) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  5. ^ Dąbrowski, Maciej. "Diagnostic Service (ClearDiagnosticInformation) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  6. ^ Dąbrowski, Maciej. "Diagnostic Service (ReadDTCInformation) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  7. ^ Dąbrowski, Maciej. "Diagnostic Service (ReadDataByIdentifier) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  8. ^ Dąbrowski, Maciej. "Diagnostic Service (ReadMemoryByAddress) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  9. ^ Dąbrowski, Maciej. "Diagnostic Service (ReadScalingDataByIdentifier) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  10. ^ Dąbrowski, Maciej. "Diagnostic Service (SecurityAccess) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  11. ^ Dąbrowski, Maciej. "Diagnostic Service (CommunicationControl) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  12. ^ Dąbrowski, Maciej. "Diagnostic Service (Authentication) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  13. ^ a b Dąbrowski, Maciej. "Diagnostic Service (ReadDataByPeriodicIdentifier) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  14. ^ Dąbrowski, Maciej. "Diagnostic Service (DynamicallyDefineDataIdentifier) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  15. ^ Dąbrowski, Maciej. "Diagnostic Service (WriteDataByIdentifier) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  16. ^ Dąbrowski, Maciej. "Diagnostic Service (InputOutputControlByIdentifier) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  17. ^ Dąbrowski, Maciej. "Diagnostic Service (RoutineControl) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  18. ^ Dąbrowski, Maciej. "Diagnostic Service (RequestDownload) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  19. ^ Dąbrowski, Maciej. "Diagnostic Service (RequestUpload) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  20. ^ Dąbrowski, Maciej. "Diagnostic Service (TransferData) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  21. ^ Dąbrowski, Maciej. "Diagnostic Service (RequestTransferExit) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  22. ^ Dąbrowski, Maciej. "Diagnostic Service (RequestFileTransfer) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  23. ^ Dąbrowski, Maciej. "Diagnostic Service (WriteMemoryByAddress) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  24. ^ Dąbrowski, Maciej. "Diagnostic Service (TesterPresent) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  25. ^ Dąbrowski, Maciej. "Diagnostic Services (AccessTimingParameter) — py-uds documentation". uds.readthedocs.io. Retrieved 2026-01-08.
  26. ^ Dąbrowski, Maciej. "Diagnostic Service (SecuredDataTransmission) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  27. ^ Dąbrowski, Maciej. "Diagnostic Service (ControlDTCSetting) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  28. ^ Dąbrowski, Maciej. "Diagnostic Service (ResponseOnEvent) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  29. ^ Dąbrowski, Maciej. "Diagnostic Service (LinkControl) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  30. ^ Dąbrowski, Maciej. "Diagnostic Message (Diagnostic Request) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  31. ^ Dąbrowski, Maciej. "Diagnostic Service (SID) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  32. ^ Dąbrowski, Maciej. "Diagnostic Message (Diagnostic Response) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  33. ^ Dąbrowski, Maciej. "Diagnostic Service (RSID) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-11.
  34. ^ a b Dąbrowski, Maciej. "Diagnostic Message (Negative Response Codes) — py-uds documentation". uds.readthedocs.io. Retrieved 2025-11-09.
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